To me it looks like a base for a Lyman 66 Peep sight.
Naw, I guess the little arrows in the window make it look more like a Redfield base. It looks more Redfield than Lyman, and similar to my Redfield 75 which is on a rimfire setup.
Rednecks... Keeping the woods critter-free since March 2, 1836. (TX Independence Day)
I'm going to use the words "clip" and "Long Colt" every chance I get. It grinds my gears to see new members attacked when we all know dang good and well what's being refered to.